Precision of natural satellite ephemerides from observations of different types

Abstract Currently, various types of observations of natural planetary satellites are used to refine their ephemerides. A new type of measurement – determining the instants of apparent satellite encounters – has recently been proposed by Morgado and co-workers. The problem that arises is which type of measurement to choose in order to obtain an ephemeris precision that is as high as possible. The answer can be obtained only by modelling the entire process: observations, obtaining the measured values, refining the satellite motion parameters, and generating the ephemeris. The explicit dependence of the ephemeris precision on observational accuracy as well as on the type of observations is unknown. In this paper, such a dependence is investigated using the Monte Carlo statistical method. The relationship between the ephemeris precision for different types of observations is then assessed. The possibility of using the instants of apparent satellite encounters to obtain an ephemeris is investigated. A method is proposed that can be used to fit the satellite orbital parameters to this type of measurement. It is shown that, in the absence of systematic scale errors in the CCD frame, the use of the instants of apparent encounters leads to less precise ephemerides. However, in the presence of significant scale errors, which is often the case, this type of measurement becomes effective because the instants of apparent satellite encounters do not depend on scale errors.